Coating compositions comprising particulate polyamides dispersed in aqueous solutions of alkali metal silicates



rosion.

United States ,Patent-O COATING COMPOSITIONS COMPRISING PARf TICULA'IE POLYAMIDES 'lDISPERSED IN AQUEOUS SOLUTIGNS F `ALKALI, METAL SILICATES f l Harry A. Toulmin,1 Jr., Dayton, Ollio,

land lChemical Corporation, poration of Delaware No.nmwing. .Application March l11, i953, ,serial N0.s41,sz4

18 Claims. (Cl. 26m-29.2

assigner to Mid- Wilmington, Del., a cor- This invention relates tolnew coating compositions for use as anti-corrosive protective coatings on metal and other surfaces. More particularly, the invention relates to `new andfnovel coating compositions comprising alkali metal `silicatesand particulate polyamides.

YProtectivecoatings `for metalsurfaces must -meet several stringent requirements. Such coatings must be continuous and impervious to `corrosive influences,` suiciently hard to withstand the abrasive and mechanical shock to which `metal objects are normally subjected in use, and must adhere firmly to the surface `on which they are deposited for protective purposes.

It has been proposed, previously, to provide coating compositions comprising an aqueous solution of sodium silicate `containing .zinc dust. It is lfound, however, that if the readily available, uhighly alkaline ycommercial grades of sodium silicate. (ratio -of sodium oxide to silica, 1:2) are used as the sole silicate constituent of the vehicle for the zinc dust, `the 4highly alkali-ne material tends to attack the zinc, with 'evolution of hydrogen, -soon after the two are mixed, and that lthe compositions `also give a considerable etllorescen'ce ofsodiumcarbonate when exposed to the air. 0n the other hand, if .sodium silicate -of :higher sodium oxide `.to silica ratio, .for example 1:3, is 'used as the sole silicate constituent of "the vehicle, there *is less tendency ifor `the silicate .to rattack the Vzinc, but thick coat ings lof the composition tend "to crack fairlyreac'lily.

rOne `object `of `this invention fis to provide Aa `coating composition comprising an .alkali `metal silicate 'and having anticorrosion properties equal to for approximating the anti-corrosion properties of such compositions containing zinclfdust `but .which `not contain zinc.

.Another-objecttdf the .invention is 4to provide `a coating composition comprising `an `alkali metalsilicate which can `be.appliedeasilyltornetal vor-other surfaces and which will adhere .strongly to the :surface to protect `it from cor# Another object is :to `provide :a ycoating `composition comprising an alkali `metal silicate which, when applied toa :metal surface, ,protects vthe flatter with -a continuous, hard, strongly adherent, ranti-corrosive rcoating which is l flexible and does `not l-tend ato :crack `when `the `:coated surfacezis subjected to .-a bending Jforce@evenwh'en the coating is comparatively thick.

These fand other objects .and :advantages of the invention willappear more clearly fr mzthe following fescription, setting forth the inventioniintdetail.

The objects `stated, land lothers, :are lfaccomp'lishei 1in `accordance with Ithis .invention by .incorporating `sa .high-` molecular-weight, resinous polyamide in particulate or dust .fform in `an Iaqueous .solution fof 1an .alkali .'metal silicate, applying Ethe `resulting composition `z to fthe .surface to be protected, and dryingtthefcoating. obtained :is hard, ranti-.carrosive, `.strongly adherent, fhea`tand fabrasionvresistant, alkali-resistant land insoluble ih water.

The polyamides which are used in the particle or dust and the diamines.

. alpha-aminoacids.

ICC

diamine and are characterized in having the amide groups as integral members of the polymer chain.

The heating may be performed in two stages. Thus, the diamine and dibasic acid may be heated to produce a relatively low molecular weight polyamide which is then subjected to further heating until a-product oli high molecular weight, or a so-called.super-polyamide, is obtained, this last-mentioned or supplementary heating step being performed under conditions suc `that volatile lay-products are removed from the mass as` they are formed. The heating to the superpolyamide stage inay be carried out in a molecular still, or the volatile by-products may be removed by the passage of .an rinert gas oran organic vapor through or over the molten polymer with or without the use of reduced pressure to carry away the volatile prodeucts, Other expedients Vfor removing `the volatiles may be employed. v

The polyamides used in particulate or dust form in practicing this invention are long-chain condensation polymers having a molecular weight of at least 5,000, preferably at least 10,000. Tfhe molecular weight may range from 5,000 `to the `molecular weight at which the polyamides are cold-drawable and exhibitber-forming properties. Preferred polyamides `are the commercially available products known .as nylon and Perlon The acids which may be condensed with the diamines to obtain thepolyarnides include carbonio, oxalic, succinic, glutaric, adipic, pimelic, .sebacic, hexadecamethylene dicarboxylic phthalic, etc. FThe .diamines which may be used include ethylene diamine, .trimethylene diamine, vPropylene diamine, pentamethylene diamine, .benzidine, phenylene diamine, etc,

By .dibasic .acids and diamines as the terms are used herein are meant not .only the dibasic acids and diamines themselves vbut .also .derivatives thereof, :inasmuch as the polyamides may he prepared directly from. .the acids plus the diamines or from some suitable derivatives .ofthe acids The polyamides may also be `derived y.by autocondensation of amino .acids .such Aas S-arnino-caproic acid, NHCHZMCODH, etc., or `from N-.carboanhydrides tof The .n amiug .of vthe Yamino acids mentioned :herein 4is in accordance Awith the systernlofv numbering given in Organic .Chemistry by .Hollemam .7th edition, 1930. By amino acids,.there.is meant `not only the amino acids themselves but also derivatives thereof.

The polyamides .may also be ,obtained by condensation of Vcyclic amides, preferably those containing `more than six annular atoms, and notably from epsilon-caprolactam. Thepolyamidesmay be obtained from `these cyclic `amides by .heating the 'amide in .the .presence of .water Aand -preferably to temperatures between about l` and 300 C., under superatrnospheric pressure, which is .usually from 200-280 pounds `per square inch, to elect -a partial polymerization, and .then converting .the low Apolymer thus-obtained to a polymer .of .high molecular weightby distiiling oil `the water gradually .from the reaction `mixture Vwith eventual :reduction of the pressure "to atmospheric pressure and athen .heating athernass at atmospheric pressure and a temperature in the range of l=80`300 C. until `the condensation polymer ofldesired molecular weight is obtained.

The particulate polyamide or polyamide dust used in the invention may be obtained 'by converting the high molecular weight polyamide toa molten state 4by heating it Pto a temperature below the 4decomposition .temperature forthe polyamide, "sdlidifying the `molten ,mass .by cooling it with solid 'carbon dioxide, and then grinding the solidilied mass to obtain particles having an average size such that at least 90% of the particles pass through a 100 to 400 mesh Tyler sieve.

The alkali silicate solution may be made up of sodium silicate, potassium silicate, lithium silicate, or a mixture of such silicates, in the form of an aqueous solution having, -for example, a specilic gravity of about l.26-1.3 (i. e.,`about 25%-30% by weight of alkali silicate). Etective coating compositions maybe made upv with alkali silicate solutions of both higher and lower specific gravity, and satisfactory results may be obtained in prac tice with solutions containing from about 20% to about 40% by weight of the alkali silicate. The sodium silicate, for example, may be any of the commonly marketed ratios of VNaOz to SiOz, which generally range from 1:1.7 to l:3.9. In general, coatings having satisfactory drying characteristics are obtained when the compositions of this invention contain a sodium silicate having a NaOz to SiOz ratio between about l :2.4 and 1 :2.6.

Mixtures of the alkali metal silicates and mixtures of the particulate polyamides may be used in the new compositions. The compositions of the invention are obtained by'mixing the polyamide particles of 100 to 400 mesh with the alkali silicate solution. The amount of particulate polyamides used in the coating composition may range from about lbs. to 18 lbs. of the polyamidel per gallon of alkali silicate solution. s

In forming a hard, protective, anti-corrosive coating on a metal surface using the compositions of this invention, the metal surface, such as iron or steel, is preferably first cleaned to remove scale, rust, dirt and the like, for instance in a pickling bath such as a solution of phosphoric acid or sulfuric acid, -followed by rinsing, or by sand blasting, but'ting or other mechanical means. The surface of the metal is then coated with the composition comprising the particulate polyamide incorporated in the aqueous alkali metal silicate solution. The composition may be applied to the surface by any appropriate method such as brushing, dipping, spraying or the like. The dipping procedure usually produces a relatively heavy coating which varies in thickness with the viscosity of the coating lcomposition, while brushing and spraying methods, being more easily controlled, are useful in providing a coating of any desired thickness ranging from very thin to heavy anti-corrosive coatings of excellent physical characteristics.

The composition applied to the metal surface is then driedunder conditions such as to substantially dehydrate it. This may be accomplished by allowing the coating to dry in the air under normal temperature and humidity conditions, by force-drying, or by a combination of airdrying and forced-drying. If forced-drying is employed, it is performed at temperatures'below the temperature at which the particular polyamide is decomposed by heat, but since the polyamides, and particularly the preferred superpolyamides have highdecomposition and melting points, relatively high drying temperatures may be used without detriment to the polyamide. Usually temperatures between 200 and 350 F. are'satisfactory. The drying may be carried out with lthe'aid of infra-red light, which requires a comparatively short heating time, only.

The compositions of the invention may be modified in certain respects to obtain special results or effects. For example, fatty acids may be incorporated in the compositions for enhancement of their spraying and adhesion' characteristics. Also, pigments and/ or llers may be included in thecompositions to provide increased body or to modify the appearance, with respect to color or otherwise, of the anti-corrosive protective coating.

The following specific examples will serve to illustrate the practice of the invention:

Example I Nylon dust (obtained by bringing commercial nylon to the molten state, solidifying the molten mass with the use of Dryv Ice, and grinding the solidified massto obtain 4 particles of which passed through a 400meshTyler sieve) is dispersed in a 30% aqueous solution of sodium silicate having a NaOz to SiOz ratio of 112.6 by slowly adding the nylon dust to the solution with constant agitation until a smooth suspension containing eight pounds of the nylon dust per gallon of silicate solution is obtained.

, Example II Perlon dust (obtained by bringing commercial Perlon-a high molecular weight polyamide from epsiloncaprolactam--to the molten state, solidifying the molten material with the aid of Dry Ice, and grinding the solid mass to obtain particles at least 90% of which pass through a -mesh Tyler sieve) is dispersed in a 35% aqueous solution of a sodium silicate having a NaOz to SiOz ratio of 1:2.3 to 1:3 by adding the dust slowly to the solution with constant agitation until a smooth suspension containing about 12 lbs. of the dust per gallon of the sodium silicate solution is obtained.

Example I Il A polyamide dust (prepared by bringing a polyamide obtained by self-condensation under heating of the amino acid G-aminocaproic acid and having a unit length of 7 and a melting point of 205 -2l0 C. to molten condition,

solidifying themolten mass with the aid of Dry Ice and grinding the solid mass to obtain particles at least 90% of which pass through a 40G-mesh Tyler sieve) is added slowly to a 30% aqueous solution of a sodium silicate having a NaOz to SiOz ratio ,of about 1:2.4, with constant stirring, until a smooth suspension containing l0 lbs. of the particulate polyamide per gallon of alkali silicate solution is obtained.

The above mixtures or compositions are applied on' articles to be coated and allowed to air-dry or to air-dryfor about ten minutes and then baked at elevated tern peratures of 200 to 300 F.

The coatings obtained-,by the application of the composition of thefinvention are extremely durable, arel not affected by rain, salt water, oxygen or vthe like, and are continuous films of excellent protective qualities.

It willbe understood that, while there have been given herein certain s pecic examples of the practice of'this invention, it is not intended thereby to have the inventionv limited to the specc details with respect to the respective' a.y molecular weightV of` at least 5,000 and an average particle size such that at least 90% of the particles are capable of passing through a 100 to 40G-mesh Tyler sieve, in an aqueous solution of from about 20% to about 40% by weight -of an alkali metal silicate having an X02 to SiO: ratio -between about 1:1.7 to 113.9, X representing the alkali metal, the amount of particulate polyamide in the composition being vbetween about 5 lbs. and 18 lbs; thereofper .gallon of the aqueous alkali metal silicate solution. I

2. A liquid coating composition as inclaim 1 in which the polyamide is the condensation product of a vdicarboxylic acid and a diamine.

3. A liquid coating'composition asin claim l in which theparticulate polyamide is polymerized epsilon aminocaproic acid.

4. A liquid coating composition as in claim 1in which the particulate polyamide is a polymerized cyclic `amide having at least six annular atoms.-

5. A liquid coating composition as in claim l` in which lactam.

composition comprising a dis- 6. A liquid coating composition as in claim l in which the alkali metal silicate is sodium silicate having an NaOz to SiOz ratio of from 1:2.4 to 1:2.6.

7. A liquid coating composition as in claim l in which the alkali metal silicate solution is a 25% to 40% solution of sodium silicate having an NaOz to SiO2 ratio of from 1:1.7 to 1:3.9.

8. The method of providing metal and other surfaces with a strongly adherent, corrosion-resistant, protective coating which comprises applying to the surface a composition comprising a particulate polyamide having the amide groups thereof as integral members ofthe polymer chain, a molecular weight of at least 5,000 and an average particle size such that at least 90% of the particles are capable of passing through a 100 to 400 mesh Tyler sieve dispersed in an aqueous solution of from about 20% to about 40% by weight of an alkali metal silicate having an X02 to SiOz ratio between about 1:1.7 to 1:39, X representing the alkali metal, the composition containing from about 5 to 18 lbs. of the polyamide per gallon of the alkali metal silicate solution, and drying the coating on the surface.

9. The method of claim in which the coating is dried in the air under normal temperature and humidity conditions.

10. The method of providing metal and other surfaces with a strongly adherent, corrosion-resistant, protective coating which comprises applying to the surface a composition comprising a dispersion of a particulate polyamide having the amide group thereof as integral members of the polymer chain, a molecular weight of at least 5,000 and an average particle size such that at least 90% of the particles are capable of passing a 100 to 400- mesh Tyler sieve in an aqueous solution of from about to about 40% by weight of an alkali metal silicate having X02 to SiO2 ratio between about 1:1.7 and l:3.9, X representing the alkali metal, and drying the coating on the surface, the proportion of polyamide to alkali silicate solution being between 5 and 18 lbs. of the polyamide per gallon of the solution.

ll. A method according to claim 10 characterized in that the particulate polyamide is the condensation product of a dicarboxylic acid andi a diamine in dust form.

l2. A method according to claim l0 characterized in that the polyamide is polymerized epsilon aminocaproic acid.

13. A method according to claim l0 characterized in that the polyamide is a polymerized cyclic amide containing at least six annular atoms.

14. A method according to claim 10 characterized in that the polyamide is polymerized epsilon-caprolactam.

l5. A method according to claim 10 characterized in that the polyamide is dispersed in a 25% to 40% aqueous solution of sodium silicate having an NaOz to SiOny ratio of from 1:1.7 to 1:3.9.

16. As a new article of manufacture, a surface having a hard, rmly adherent, anti-corrosive coating thereon comprising an alkali metal silicate having an X02 to SiOz ratio between 1:1.7 and 1:3.9, X representing the alkali metal and a particulate polyamide having the amide groups thereof as integral members of the polymer chain, a molecular weight of at least 5,000 and an average particle size such that at least of the particles are capable of passing through a to 400 mesh Tyler sieve.

17. As a new article of manufacture, a surface having a hard, firmly adherent, anti-corrosive coating thereon comprising a sodium silicate having an NaOz to SiOz ratio of from 1:1.7 to 1:3.9 and a polyamide which is the condensation product of a dicarboxylic acid and a diamine in dust form.

18. As a new article of manufacture, a surface having a hard, firmly adherent, anti-corrosive coating thereon comprising a sodium silicate having `a NaOz to SiOz ratio of form 1:1.7 to 1:3.9 and a high molecular weight polymerized epsilon-caprolactam, in dust form.

References Cited in the le of this patent UNITED STATES PATENTS 2,467,192 Cupery Apr. 12, 1949 

1. A LIQUID COATING COMPOSITION COMPRISING A DISPERSION OF A PARTICULATE POLYAMIDE HAVING THE AMIDE GROUPS THEREOF AS INTEGRAL MEMBERS OF THE POLYMER CHAIN, A MOLECULAR WEIGHT OF AT LEAST 5,000 AND AN AVERAGE PARTICLE SIZE SUCH THAT AT LEAST 90% OF THE PARTICLES ARE CAPABLE OF PASSING THROUGH A 100 TO 400-MESH TYLER SIEVE. IN AN AQUEOUS SOLUTION OF FROM ABOUT 20% TO ABOUT 40% BY WEIGHT OF AN ALKALI METAL SILICATE HAVING AN XO2 TO SIO2 RATIO BETWEEN ABOUT 1:1.7 TO 1:3.9, X REPRESENTING THE ALKALI METAL, THE AMOUNT OF PARTICULATE POLYAMIDE IN THE COMPOSITION BEING BETWEEN ABOUT 5 LBS. AND 18 LBS. THEREOF PER GALLON OF THE AQUEOUS ALKALI METAL SILICATE SOLUTION. 